1. Field of the Invention
The present invention relates to high temperature fuel cell generators wherein depleted fuel and depleted air are kept separate from each other to allow treatment of depleted fuel by a special apparatus to separate essentially pure carbon dioxide from depleted fuel.
2. Background Information
Tubular, solid oxide electrolyte fuel cell xe2x80x9cSOFCxe2x80x9d generators have been well known in the art for almost twenty years, and taught, for example by A. O. Isenberg in U.S. Pat. No. 4,395,468. There, in the main embodiment, air, as oxidant, reacted at the inside xe2x80x9cairxe2x80x9d electrode of a closed tubular SOFC, to yield depleted air; and fuel, such as CO and H2, reacted at an outside xe2x80x9cfuelxe2x80x9d electrode of the closed tubular SOFC to yield depleted fuel, all in a xe2x80x9cgenerating chamber,xe2x80x9d at high temperatures (for example, about 1000xc2x0 C.). The air electrode generally comprised a doped lanthanum manganite, the fuel electrode generally comprised a nickel cermet and an electrolyte disposed between the electrodes generally comprised a stabilized zirconia. The depleted fuel was subsequently completely combusted with the depleted air in a separate combustion chamber, to preheat feed air. This basic SOFC generator design was carried forward, with other improvements, as shown for example in U.S. Pat. Nos.: 4,664,986; 5,573,867; and 5,733,675 (Draper et al.; Zafred et al. and Dederer et al.). Other designs have used a series of fuel cell stacks, each providing a stage containing a different electrolyte operating at a lower temperature to improve fuel gas utilization, as taught in U.S. Pat. No. 5,712,055 (Khandkar). In a somewhat similar fashion, in one embodiment of U.S. Pat. No. 5,134,043 (Nakagawa), xe2x80x9cdepleted fuelxe2x80x9d from a first molten carbonate fuel cell system was sent to a separate molten carbonate anode, where the product was then mixed/contacted with oxidant/air before being introduced into the cathode section of the first molten carbonate electrolyte fuel cell. While tubular fuel cells are emphasized herein, flat/planar fuel cells, which are well known in the art, may also be used.
However, such designs could release byproducts of combustion, such as carbon dioxide into the atmosphere. Efforts are now being made on an international level to globally reduce the release of so-called xe2x80x9cgreen house gassesxe2x80x9d which includes carbon dioxide, which may contribute to global atmospheric warming. Such efforts may, indeed, lead to future legislation regarding carbon dioxide emissions from SOFC""s. What is needed is a means to further treat the spent fuel from fuel cell generators to not only reduce or eliminate carbon dioxide emissions but also to increase the capacity of the fuel cell generators to further utilize feed fuel. Such a need applies to both tubular and flat plate type fuel cells.
In the area of reducing carbon dioxide emissions from power plants utilizing a variety of types of fuel cells, in order to reduce the xe2x80x9cgreen house effectxe2x80x9d, U.S. Pat. No. 4,751,151 (Healy et al.) taught a carbon dioxide absorber, such as monoethanolamine, included as a regenerable carbon dioxide absorbent, for stripping carbon dioxide, followed by subsequent cooling and compression. U.S. Pat. No. 5,064,733 (Krist et al.), recognizing prior art conversion of natural gas into DC electricity plus carbon dioxide and water-with the accompanying creation of a DC electrical power-in a solid oxide fuel cell taught conversion of the carbon dioxide and water to C2H4, C2H6 and C2H2 by use of a copper, copper alloy or perovskite cathode. That cathode was in contact with the CO2, and H2O and a dual layered anode made of metal oxide perovskite next to the electrode with an outer contacting layer of rare earth metal oxide contacting CH4. This provided for concurrent gas phase electrocatalytic oxidative dimerization of methane at an anode on one side of a solid electrolyte and reduction of carbon dioxide to gaseous hydrocarbons at a cathode on the other side of the solid electrolyte. Other CO2 treatments include U.S. Pat. No. 5,928,806 (Olah et al.), where a regenerative fuel cell system containing two electrochemical cells in fluid communication were taught, one cell oxidizing an oxygenated hydrocarbon, such as methyl alcohol, formic acid, etc., to CO2 and H2O and a second cell reducing CO2 and H2O to an oxygenated hydrocarbon. This produced methyl alcohol and related compounds directly from CO2. Also, U.S. Pat. No. 5,866,090, (Nakagawa et al.) taught reacting carbon dioxide containing exhaust, from a power plant which uses fuel cells, with a composition containing lithium zirconate at approximately 450xc2x0 C., so that the carbon dioxide reacts with the lithium zirconate to produce lithium carbonate and zirconia. The lithium carbonate and zirconia are then subjected to a temperature of 600xc2x0 C. or more, so as to produce lithium zirconate and pure carbon dioxide.
While a great many methods to treat carbon dioxide are known, a new fuel cell generator design is needed to allow segregation and further concentration of the carbon dioxide for such treatment.
Therefore it is a main object of this invention to provide an improved fuel cell generator design, allowing segregation of carbon dioxide generated at the fuel electrodes by an integrated secondary fuel reactor means.
It is a further object of this invention to provide an improved generator design allowing ultra high fuel utilization capacity by use of an integrated secondary fuel reactor means.
These and other objects are accomplished by providing a high temperature fuel cell generator comprising a separate generator chamber containing tubular solid oxide electrolyte fuel cells which operate on oxidant and fuel to yield depleted oxidant and depleted fuel, and a separate fuel reactor chamber containing a depleted fuel reactor-selected from a fuel cell and an electrolysis cell, and potentially operating at a different temperature than the generator chamber-where all oxidant and fuel passages are separated and do not communicate directly with each other, so that fuel and oxidant remain effectively separated, where a depleted fuel exit is provided in the separate fuel reactor chamber for exiting a gas consisting essentially of carbon dioxide and water for further treatment, and where depleted oxidant exits are provided to exhaust to the environment.
The invention also comprises a high temperature fuel cell generator, comprising: a housing defining and separating a generator chamber; a separate fuel reactor chamber; a depleted oxidant discharge chamber; a plurality of fuel cells, each having an electrolyte contacted on one side by an air electrode and on the other side by a fuel electrode, said fuel cells disposed within the generator chamber; means to react substantially all of the fuel, said means selected from oxidant-fed fuel cells and steam-fed electrolysis cells disposed in the separate fuel reactor chamber; means to flow a feed fuel gas to contact the fuel electrode of fuel cells in the generator chamber, where said fuel can react and provide partially depleted fuel gas; means to flow a feed oxidant gas to contact the air electrode of fuel cells in the generator chamber, where said oxidant can react and provide a depleted oxidant gas; means to flow a second feed oxidant gas to the separate fuel reactor chamber, where the means to react substantially all of the fuel is a fuel cell, or means to flow steam to the separate fuel reactor chamber, where the means to react substantially all of the fuel is an electrolysis cell; and means to flow partially depleted fuel gas from the generator chamber to contact the outside of the means to react substantially all of the fuel in the separate fuel reactor chamber, where said depleted fuel can further react and provide a completely depleted fuel gas consisting essentially of carbon dioxide and water, where depleted oxidant gases are kept separated from all depleted fuel gases and said depleted oxidant gases flow into a separate depleted oxidant discharge chamber.
The invention also covers a method of operating a high temperature fuel cell generator comprising a separate generator chamber and a separate fuel reactor chamber, the generator chamber containing oxidant-fed tubular solid oxide fuel cells and the separate fuel reactor chamber containing means to react substantially all of the fuel, selected from oxidant-fed fuel cells and steam-fed electrolysis cells, comprising the steps of: (1) feeding feed fuel gas to contact fuel electrodes of fuel cells in the generator chamber to provide partially depleted fuel gas, (2) feeding partially depleted fuel gas to contact the outside of the means to react substantially all of the fuel in the separate fuel reactor chamber, to provide completely depleted fuel gas consisting essentially of carbon dioxide and water, (3) feeding feed oxidant gas to fuel cells in the generator chamber to provide depleted oxidant gas, and (4) feeding a second feed oxidant gas to contact fuel cells in the separate fuel reactor chamber or steam to contact electrolysis cells in the separate fuel reactor chamber; where oxidant gases are kept separated from all fuel gases.
Thus, this invention relates to an arrangement of components within a fuel cell generator by means of which the exhausted CO2 and H2O are sequestered by separate fuel depletion means to react substantially all of the fuel, where such means can be extra banks of fuel cells or, preferably, electrolysis cells. By this means, the water can be condensed from the product exhaust stream, and the carbon dioxide can be pressurized or liquefied and put to use, rather than being released to the atmosphere. Consequently the release of a xe2x80x9cgreen house gasxe2x80x9d pollutant is avoided. The main idea is to produce electric power and sequester carbon dioxide. The secondary fuel reactor means-either a set of power-producing fuel cells or a set of electrolysis cells running on steam-are designed to operate at very high fuel utilization. In the arrangement of this invention the main generator and the depleted fuel reactor sections are conveniently integrated within a common enclosure. The common enclosure can also contain steam inlet means to provide steam to heat feed oxidant and pre-reform feed fuel, as well as to feed a set of electrolysis cells. Stack reformer passages can also be positioned between the fuel cells in the generating chamber. Preferably, the fuel cells used in either the generator section or the depleted fuel reactor section are of the well known tubular type.